While it's true that in terms of light/area (and thus exposure) that f/2.8 is f/2.8, that is only regarding the light per area. Because a full frame lens delivers that same flux over an area four times the size of a m43 lens, in terms of total light captured the full frame lens has a two stop advantage.

This makes even more intuitive sense when you consider the definition of aperture... Focal length / diameter. If you double the focal length, then you halve the aperture.

Finally, if you still choose to not believe me, then believe metabones: their speed booster increases the speed by 1 stop and decreases the focal length by 0.71. Apply twice for m43, and you get +2 stops and half focal length. That means a 24-70mm f/5.6 with 2 boosters becomes a 12-35 f/2.8.

Thus, in reverse, Panasonic's 12-35mm f/2.8 is equivalent to a 24-70mm f/5.6 on full frame. Indeed, we don't even have to work entirely in the hypothetical on that: consider applying a 2x teleconveter to the 12-35mm. It enlarges the image circle by a factor of 2, thus making it full frame sized. Same lens, same size and weight, just an adapter away. There is no reason to believe that the optics couldn't be refactored to include the adapter's effect, thus a Full Frame 24-70mm f/5.6 would be, to a decent approximation, the same size and weight as Panasonic's 12-35mm f/2.8.